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The question is excellent, and it would take part of a book, or at least a good paper, to answer it thoroughly. And I do try to ‘teach’ that subject in my forthcoming work. Approximately, there are about a dozen-plus groups (usually recognized as sections) of related species of Agaricus in the temperate latitudes, and many more towards the tropics. Yes, A. californicus belongs in section Xanthodermatei. A. bitorquis belongs in section Bivelares and A. bernardi belongs in section Chitonioides. Arvenses is yet another section. There are tables of characters useful for separating the section-ranked groups with more or less success, and odor is one very useful character if it works for you. But there are overlaps (shared characteristics), exceptions to generalizations, etc. Ultimately, DNA sequencing has clarified some of the more difficult and unusual cases. Several such groups including newly discovered ones have only 1 or 2 species apiece in the USA and Canada.

Species of unknown edibility that are closely related to established edible species may be regarded (in one reasonable approach) as ‘reasonable to try with the usual cautions’. But here you must also know your phylogeny. In the example you cite, this could apply to A. pearsonii. But you mention A. bitorquis and A. bernardi, actually in separate sections, and the next most closely related section to those two is… Xanthodermatei. As usual, there is no approach that doesn’t require at least some degree of experience in order to avoid pitfalls.

Thanks, Rick. I’m rather new to this but excited to learn: So this new species belongs to the toxic section of Agaricus genus called Xanthodermatei? Along with say, A. californicus? And edibles like A. bitorquis or A. bernardi belong to the generally edible section called Arvense? So are there means other than odor to differentiate between the two sections? For example,I was looking at Genbank and ran blast on A. pearsonii as compared to A. bernardi (also stains red) and A. bitorquis and they all appear to closely related? Can one make any inferences about the edibility of A. pearsonii as a result? Sorry if this is a totally naive question.
Terri

Hi — Yes, a formal description and name for this new species is provided in a book manuscript that it at the publisher now. I have not been given a publication date yet. More than 40 new names will go active when the book is released. Until then, it is not a good idea for them to come into use.

For your info, this toxic species belongs to section Xanthodermatei, comprising 20+ species in North America, of which seven will be named for the first time in the book. Mycologists have had a hard time distinguishing among many of these species. This one (Dan’s Ohio specimen) is known from the Smokies into Quebec, and westward past the Great Lakes region.

Rick sent me an email a couple of months ago, but I’m not sure if the new name is published yet. It was going to be Agaricus pallens, but he dropped that name in favor of an epithet which refers to the mushroom’s slender stature. The new name is not yet showing up on a google search, so I guess it’s still secret for now;)

I forgot (!) to mention that when I opened the well-wrapped and sealed packet of dried specimens that Dan sent, the next thing I did was to smell them. There was a distinct, though faint, odor that I recognized as ‘typically phenolic’ (not really unpleasant or pleasant to me, but a big red flag for edibility). Dried Agaricus mushrooms generally lose their fresh odors but sometimes the odor will linger for a while. So there’s another datum on the odor of this collection.

The (woodland) Agaricus that I have always avoided collecting for the table has what I think of as a chemical or medicinal odor. Excellent point about olfactory variability amongst people. After having repeated a certain sensory experience over the course of time, one tends to develope the notion that this may be objectified. Observing a spore print color can be another such sensory experience.

But I’ll go one step further. Viewing pictures of fungi and then hoping to make a stab at the ID can be fraught with sensory traps. Having reviewed this discussion it occurs to me that Dan had not reported a “pleasant odor” until after I had mentioned this likelihood within one of my posted comments. So,at the time I had made that comment, that data was not yet available. I must have viewed those first two pics (dark squamules hard to see) and experieced an olfactory hallucination… at least in terms of how I later remembered the experince of viewing the pics. They reminded me so much of my own experience of finding A. abruptibulbus that I “completed” the experience by attaching an odor. (!)

First, each person’s perception of odor (as with the sense of taste) is personal and idiosyncratic. At least one 20th century author never in 50 years encountered an Agaricus odor that wasn’t pleasant or mild (to him), and he had sniffed several dozen species. Second, various factors including age, stage and condition influence production of odor compounds. Third, chemical concentration affects the character of odors as perceived. Finally… these toxic mushrooms have various ‘phenolic’ odors ranging from that of ‘library paste’ in A. californicus (a pleasant odor to my nose) through creosote-like and the ‘gall-nut-ink’ of older French authors into truly stinking phenol and/or hydroquinone odors and even of ammonia and iodine in the case of A. iodosmus. It’s very useful to train the nose to recognize and associate all of these diverse but related odors, especially if you eat wild Agaricus.

a curious species was collected and photographed. Folks debated the ID, then an expert was able to jump in while the mushroom was still fruiting in the field and obtain specimens and an eventual species determination for a rarely collected and currently undescribed species. Amateur collector and expert, working together, contributed to the furthering of mycological taxonomy. And YOU were there!

Here’s an update: I sequenced Dan’s collection ’MO 24015" from this patch and it matches the undescribed species already known from North Carolina. The spore size is also a reasonable match. I have a provisional new name for this species that I intend to publish in the next year or two. It belongs to the toxic section Xanthodermatei and should not be confused with the edible A. abruptibulbus.

I’d wager a tidy sum that this is an undescribed species in the toxic section Xanthodermatei, rather than in the generally-edible section Arvenses. It has ‘the look’, for example the dark brown disc and fibril pigment (rare in the Arvenses group). It is quite similar to a (another?) paler undescribed species known from Tennessee (with two sequenced specimens). If you go to the GenBank database, you will also find a slightly variant sequence obtained from a soil sample (in the absence of mushrooms) in an experimental vegetation plot in Minnesota (see GenBank DQ421108). So, from the upper midwest we have a sequence, and these photos, but no specimens tie it all together… yet.

I’d enjoy having a chance to evaluate specimens like these (great photos!) from Ohio and would cheerfully comment further if some could be sent.

A. pocillator (with a cup-shaped stipe base) and A. placomyces (in the A. H. Smith sense) are cousins in the same toxic group (if I’m right).

Ok, Dan, next time you go up that hill go look for it. These tend to
be repeated fruiters. The pink above the annulus is a very common
Agaricus feature. The odor of the Arvenses is not always apparent and
some scratching may be necessary. The dark fibrillose squamules on the
cap is what bothers me for a member of the Arvenses group. Also try
for an odor of phenol in the base of the stem as there are some pretty
lean members of that group too.

reaction is sometimes present, sometimes not… as I recall my own collections. The darkening of the center of the pileus is also a “sometimes, sometimes not” feature. I don’t recall any collections which did not exhibit the pleasant odor. I have no clue why the original author did not include this. The manual where I first learned A. abruptibulbus was the Audubon, which is currently packed up with my vacation gear. Roody and B/B/F each mention the almond odor (using the two different species names).

what makes this esteemed group of identifiers even consider this
species to be in Section Arvenses? The original author does not
stipulate neither odor, not yellowing reaction, which are the key
attributes for that Section. The photos reveal none of the
second, which usually is present at least a little bit. Moreover,
I see a dark squamulose pileus in there, which sends us elsewhere
in the Agaricus book. Agaricus is a rich and complex Genus that
in parts of North America has not been fully written up yet.

collected A. abruptibulbus for the table (when I see it, generally one or two good specimens if lucky) now for about 18 years. There does seem to be a developmental stage (perhaps short lived, I have seen it) when the gills are a vivid pink. Maybe not every specimen gets it. But what else could this slender, elegant, pleasantly perfumed woodland Agaricus be besides one or the other of A. abruptibulbus/ A. silvicola? I have always thought of A. silvicola as a more robust mushroom. Manuals now list the two names as synonymous (Roody for instance).

silvicola var. silvicola is listed as a synonym for abruptibulbus.
could an abrupt bulb on one just be a variety of the same thing?

more concerning for a quick ID is that BRIGHT pink gill color…lighting artifact, or is it really that bright?

neither silvicola nor abruptibulbus are described as having bright pink gills, just pinkish-brown. the bright gill color is closer to campestris (definitely not your mushroom here, Dan) and the European A. comtulus (also slender, fragrant, yellowing, but with a much more insubstantial partial veil).

It’s definitely something cool, beautiful and according to Dan’s nose, edible too! But what it is…??

it is common for either of A. abruptibulbus or A. silvicola to undergo gill color changes that begin with almost white, transition to a grayish, then to the pink, and finally to brown. These two species may actually be the same (according to several authors). The one with the tall slender stem and small bulb at the stem base is the one I have called abruptibulbus. The odor of almonds or anise is often even stronger than in A. arvensis. This is common mushroom of the eastern US woodlands. I have found it many times. Very young specimens, with the almost white gills, can easily be mistaken for Amanitas.